1. Field of the Invention
The present invention relates to an aggregate of metal oxide fine particles suitable as photovoltaic materials, photoconductive materials, and photocatalytic materials and a method for producing the aggregate. The present invention also relates to a photosensitive semiconductor electrode preferably used for photocells, photoconductive devices, display devices, and various sensors using the aggregate and a method of manufacturing the photosensitive semiconductor electrodes.
2. Prior Art
Global warming caused by the burning of fossil fuels and an increased energy demand arising from the increasing population are an important theme relating to human life and death. Sunlight, needless to say, has grown the environment of this planet and has been an energy source for all life including mankind on this planet since the beginning of life. Considerable research is being carried out on how to exploit sunlight as a clean energy source which is inexhaustible and produces no toxic substances. Particularly so-called solar cells which convert optical energy to electric energy have lately attracted considerable attention as a powerful technology. As a photovoltaic material for the solar cells, a monocrystal, polycrystal, or amorphous silicon, or a semiconductor compound such as CuInSe, GaAs, or CdS is used. Solar cells using these inorganic semiconductors exhibit the relatively high energy conversion efficiency as high as 10% to 20%. These solar cells are therefore widely used as power sources in remote locations or as auxiliary power sources for portable small electronic instruments.
However, if the aim is to prevent further harm to the Earth's environment by curbing the consumption of fossil fuels, as was mentioned above, then, at the present time, it cannot easily be said that solar cells using inorganic semiconductors are sufficiently effective. This is because solar cells using these inorganic semiconductors are produced by a plasma CVD method or a high temperature crystal growth process and the production of these devices requires a lot of energy. Further, these devices contain components such as Cd, As, and Se which are likely to harm the environment, so that there is the fear of the environment being harmed by waste devices.
In order to solve these problems, a photoelectrochemical energy conversion equipment is awaited which makes use of a photoelectrochemical reaction arising in the boundary between a photosensitive semiconductor (a semiconductor wherein carriers are generated by irradiation with light) and an electrolytic solution. Fujishima and et al. found that, when a titanium oxide electrode in an aqueous solution is irradiated with light, oxygen and hydrogen are produced by the decomposition of water while a photoelectric current flows between the titanium oxide electrode and a platinum electrode as the counter electrode (A. Fujishima, K. Honda, Nature, 238, 37 (1972)). This photoelectrochemical energy conversion equipment is attracting considerable attention since it converts solar energy into electric energy and decomposes water, which is an inexhaustible natural resource, to generate hydrogen whose use as a clean fuel is anticipated.
Titanium oxide is photoelectrochemically stable and has excellent characteristics for use as the material for photosensitive semiconductor electrodes in such an equipment. On the other hand, titanium oxide has the drawback of possessing a band gap as large as 3.0 eV and has hence poor spectrum matching with solar light whereby the photoelectric conversion efficiency of the equipment is reduced.
In order to improve the characteristics of titanium oxide, a process in which an organic dye is adsorbed by the surface of the titanium oxide thereby sensitizing the titanium oxide is being examined (H. Tsubomura, Sol. Energy, 21, 93 (1978)). There is also a proposal in which titanium oxide with a large specific surface area is used as the material for a photosensitive semiconductor electrode, based on the observation that only a dye adsorbed by the surface contributes to an increase in the sensitivity, and a sensitizing dye is allowed to be adsorbed by the surface of titanium oxide (Japanese Patent Application Laid-Open (JP-A) No. 1-220380).
A metal alkoxide hydrolysis colloidal method is proposed as a means of preparing a metal oxide thin film having a large specific surface area (Japanese Patent ApplicationLaid-Open (JP-A) No.3-114150). In this method, an excessive amount of water is added to an alcohol solution of a metal alkoxide in the presence of a deflocculant, for example nitric acid, which is added to stabilize the dispersion, which is then heated to hydrolyze the metal alkoxide thereby preparing a colloidal solution in which fine particles of a metal oxide are dispersed. The colloidal solution is then applied to an appropriate substrate which is then sintered to obtain a film of metal oxide fine particles. This method makes it possible to produce a film wherein titanium oxide ultra fine particles with a grain size of tens of nanometers are deposited. However, since the clearances between particles are smaller than the grain size of the particles, though the specific surface area of the titanium oxide fine particles forming the film is large, the sensitizing dye cannot penetrate into the inside of the film and the adsorption of the dye reaches its saturation limit immediately if the film is thick, giving rise to the problem of insufficient adsorption of the dye.
There is also a proposed method in which from 1 mol to 2 mols water is added for each mol of titanium alkoxide used as a raw material, the mixture is heated so as to prepare a transparent sol in which titanium alkoxide is partially hydrolyzed, and polyethylene glycol and the like are mixed with the transparent sol, which is then sintered to form a titanium oxide thin film having pores on its surface (Japanese PatentApplication Laid-Open (JP-A) No. 8-099041). This method, however, has the drawbacks that apart from the areas of the pores the film becomes denser and hence the amount of dye adsorbed is small, and that the thickness of the film which can be formed by one application is thin (0.05 .mu.m or less). It is necessary to repeat the application and sintering process many times in order for a sufficient amount of the dye to be absorbed. This requires complicated production steps bringing about a long production time and a large production cost.